Porous membranes made of a thermoplastic resin have been widely used, for example, as a material for separation, selective permeation, and isolation of substances. For example, they have been used as a battery separator used in a lithium ion secondary battery, nickel-hydrogen battery, nickel-cadmium battery, and polymer battery; a separator for an electric double layer capacitor; various filters such as a reverse osmosis filtration membrane, ultrafiltration membrane, and microfiltration membrane; moisture-permeable waterproof clothing; a medical material; and the like. In particular, polyethylene porous membranes and polypropylene porous membranes have been suitably used as a separator for a lithium ion secondary battery or have been under development. The reason is that they are not only characterized by excellent electrical insulating properties, having ion permeability by electrolyte impregnation, and excellent electrolyte resistance/oxidation resistance, but also have such a pore-blocking effect that excessive temperature rise is suppressed by blocking a current at a temperature of about 120 to 150° C. in abnormal temperature rise in a battery. However, when the temperature continues to rise for some reason even after pore blocking, membrane rupture can occur at a certain temperature due to decrease in viscosity of a molten polyethylene or polypropylene constituting the membrane and shrinkage of the membrane. In addition, when left at a constant high temperature, membrane rupture can occur after the lapse of a certain time due to decrease in viscosity of a molten polyethylene or polypropylene and shrinkage of the membrane. This phenomenon is not a phenomenon that occurs only when polyethylene or polypropylene is used, and also when other thermoplastic resins are used, this phenomenon cannot be avoided at or higher than the melting point of the resin constituting the porous membrane.
In particular, a separator for a lithium ion secondary battery is highly responsible for battery properties, battery productivity, and battery safety, and required to have excellent mechanical properties, heat resistance, permeability, dimensional stability, pore-blocking property (shutdown property), the property of preventing melt rupture of a membrane (meltdown prevention property), and the like. With regard to batteries for electric vehicles or hybrid vehicles, lithium ion batteries, the capacity of which can be expected to increase in the future, have been under development; meanwhile, various studies to improve heat resistance, for example, by laminating a heat-resistant resin on a polyolefin porous membrane have been conducted until now because tough mechanical strength, compression resistance, and heat resistance are required. However, in general, when a heat-resistant resin is laminated on a polyolefin porous membrane, the heat-resistant resin layer can peel off during processing of a composite porous membrane, in a slitting step, or in a battery assembly process, in which case it is difficult to ensure safety.
Further, to deal with cost reduction, it is expected that the speed will be faster in a battery assembly process. Thus, the present inventors presume that, for such a high-speed processing as well as for ensuring safety of a battery, troubles such as peeling off of a heat-resistant resin layer are required to be reduced, and even higher adhesion is necessary for that purpose.
Patent Document 1 exemplifies a separator obtained by coating aromatic polyamide (poly(phenylene terephthalamide)) containing Al(OH)3 directly to a polypropylene (PP) microporous membrane subjected to a corona discharge treatment. Patent Document 2 exemplifies a separator for a lithium ion secondary battery obtained by direct application of a polyamide-imide resin to a polyolefin porous membrane so that the membrane thickness is 1 μm and immersion in water at 25° C., followed by drying.
As in the cases of Patent Document 1 and Patent Document 2, in the roll coating method, die coating method, bar coating method, blade coating method, and the like which are commonly used when coating a coating solution directly to a polyolefin porous membrane, infiltration of a resin component into a polyolefin porous membrane is unavoidable because of the shearing force, and significant increase in air resistance and decrease in pore-blocking function occur. In such direct coating methods, a resin component readily fills the inside of pores, consequently causing extreme increase in air resistance. In addition, such methods have a problem in that membrane thickness unevenness of a polyolefin porous membrane is likely to lead to membrane thickness unevenness of a heat-resistant resin layer, which in turn is likely to lead to variation in air resistance.
Patent Document 3 exemplifies an electrolyte-supported polymer membrane obtained by immersion of a nonwoven comprising aramid fibers in a dope containing a vinylidene fluoride copolymer which is a heat-resistant resin, and drying.
Patent Document 4 exemplifies a composite porous membrane obtained by immersion of a polypropylene porous membrane in a dope mainly composed of polyvinylidene fluoride which is a heat-resistant resin, followed by the steps of coagulation, washing with water, and drying.
When a nonwoven comprising aramid fibers is immersed in a heat-resistant resin solution as in Patent Document 3, a heat-resistant porous layer is formed inside and on both surfaces of the nonwoven, and accordingly most of the continuous pores inside the nonwoven will be blocked; consequently, significant increase in air resistance cannot be avoided, and besides a pore-blocking function that determines safety of a separator cannot be obtained.
Also in Patent Document 4, a heat-resistant porous layer is similarly formed inside and both surfaces of a polypropylene porous membrane, and as in Patent Document 3, significant increase in air resistance cannot be avoided; besides it is difficult to obtain a pore-blocking function.
Patent Document 5 discloses a separator having a heat-resistant porous layer comprising para-aramid obtained in such a manner that, when a para-aramid resin solution which is a heat-resistant resin is applied directly to a polyethylene porous film, the polyethylene porous film is impregnated in advance with a polar organic solvent used in the heat-resistant resin solution in order to avoid significant increase in air resistance, and after the heat-resistant resin solution is applied, the polyethylene porous film is made into a white opaque membrane in a thermo-hygrostat set at a temperature of 30° C. and a relative humidity of 65%, and then washed and dried.
In Patent Document 5, there is no significant increase in air resistance, but adhesion between the polyethylene porous film and the heat-resistant resin is extremely low, and it is difficult to ensure safety.
Patent Document 6 discloses a composite porous membrane obtained in such a manner that a polyethylene film is coated with a polyamide-imide resin solution and passed through an atmosphere at 250° C. and 80% RH over 30 seconds to obtain a semi-gel like porous membrane; then a polyethylene porous film with a thickness of 20 μm or 10 μm is laminated on said semi-gel like porous membrane, immersed in an aqueous solution containing N-methyl-2-pyrrolidone (NMP), and then washed with water and dried.
In Patent Document 6, there is no significant increase in air resistance, but adhesion between the polyethylene porous film and the heat-resistant resin is insufficient, and the polyethylene porous film was softer than a polypropylene resin porous membrane and had poor mechanical strength and compression resistance.
As described above, in a composite porous membrane in which a heat-resistant resin layer is laminated on a polyolefin porous membrane or the like that serves as a substrate membrane, the rising range of air resistance widens if the heat-resistant resin is infiltrated into the porous membrane that serves as a substrate in order to improve adhesion of the heat-resistant resin layer. On the other hand, if infiltration of the heat-resistant resin is reduced, the rising range of air resistance can be kept small, but adhesion of the heat-resistant resin layer will be low. Taking into account speeding up in a battery assembly process, the demand for safety will be increasingly greater, but it is difficult to ensure the safety if the adhesion is low. In particular, when a polypropylene-based resin porous membrane obtained by the stretching pore-forming process was used as a porous membrane substrate, it is, in general, extremely difficult to obtain adhesion to a heat-resistant resin layer, and there was not a composite porous membrane that simultaneously provides adhesion of a heat-resistant resin layer and a rising range of air resistance.